1. Field of the Invention
The present invention relates generally to the fabrication of semiconductor devices and, more particularly, to improving the fabrication of spacers by enhancing the etch endpoint detection of a nitride layer.
2. Description of the Related Art
As is well known, the semiconductor manufacturing process involves several stages during which millions of transistors including source/drain diffusion regions, a conductive polysilicon gate, and a dielectric gate oxide are fabricated on a single semiconductor chip. One of such fabrication stages is the silicon nitride spacer etch process during which a conformal layer of silicon nitride is deposited on a surface of the substrate having fabricated transistors. This silicon nitride layer is subsequently etched utilizing a plasma etch process, creating silicon nitride spacers alongside the polysilicon gates.
As the demand for scaling down the integrated devices and thus feature sizes such as spacers is continuously increasing, so is the need for implementing a more controllable etch process. However, this need is specifically more pronounced during the one step silicon nitride spacer etch process as the aggressive nature of the etch chemistry implemented inevitably results in gate oxide punch through leading to changes in gate oxide dimensions. As is well known, changes to the gate oxide are disfavored since they modify the electrical conductivity of the gate oxide and thus the transistor.
Predominantly, the silicon nitride layer is removed through implementing a one-step plasma etching method (e.g., dry etching). The plasma etching process is typically performed in a plasma chamber in which strong electrical fields cause high energy gases containing positively charged ions and negatively charged electrons to be accelerated toward the exposed surface of the silicon nitride layer. During the one-step plasma etching process, the exposed layer of silicon nitride is chemically and physically removed as a result of being bombarded with positive ions.
Preferably, the one-step etching process of the silicon nitride layer must stop once it has been determined that the silicon nitride material has been etched through and removed from over the surface of the substrate. Additionally, this one-step etch process must be achieved without damaging the underlying layer. Consequently, to accommodate this goal, it is imperative to implement an endpoint detection method capable of stopping the one-step etch process once the silicon nitride layer has been etched through.
Thus far, optical emission spectroscopy (OES) method has been primarily utilized for detecting the etch endpoint. In this method, the light emitted by the gases within the etch reactant chamber is used to identify the specific material being etched. As the light emission intensity is directly proportional to the concentration of a specific gas within the etch reactant chamber, the endpoint detector can in theory determine when the etching of the silicon nitride material has concluded.
Thus far, however, the OES method has proven to be less than reliable and efficient etch endpoint detection as it causes the overetching or underetching of the silicon nitride layer, rendering the outcome of the one-step etch process unpredictable. Specifically, this occurs due to the variation in thickness of the silicon nitride layers between wafers of the same lot as well as the wafers of different lots. For instance, the overetching of the silicon nitride layer during the one-step etch process using the predominantly used aggressive chemistry causes the removal of portions of the gate oxide or ultimately, in gate oxide punch through.
Unfortunately, the unreliability and unpredictability associated with the OES method has a severe negative impact on fabrication stages and thus semiconductor manufacturing. Among others, the unpredictability mandates the close monitoring of the one-step etch process, multiple inspections of the semiconductor substrate during the one-step etching operation, necessity to recalibrate the tools for each substrate within the same lot as well as different lots, thus needlessly wasting valuable time, slowing down the production, yield loss, and ultimately semiconductor substrate throughput.
In view of the foregoing, a need exists for a spacer etch endpoint detection methodology and apparatus that eliminates the unpredictability and unreliability associated with the conventional optical emission spectroscopy (OES) etch endpoint detection method used during the one-step etch process while increasing semiconductor substrate throughput.